It is known in the art to prepare cross-linked and cross-linkable silicone elastomers which are mainly comprised of two types of liquid polysiloxanes; one having vinyl (or general C═C) attached to the main chain and the other having hydrogen directly attached to silicone atoms. The addition curing (or crosslinking) reaction to form elastomeric materials occurs through hydrosilation in the presence of platinum or other metal-containing catalysts under room temperature or heating conditions. The substitution groups attached to the silicone chain can be methyl, phenyl, or fluoroalkyl (mostly trifluoropropyl). The fluoroalkyl substituted silicone (fluorosilicone) generally displays good chemical resistance and is commonly used as coating, potting or encapsulation material to protect electric/electronic components and assemblies.
The preparation of curable organosilicon prepolymers or cross-linked thermosetting polymers through a hydrosilation reaction of polycyclic polyene (providing an active C═C) and reactive cyclic polysiloxane or tetrahedral siloxysilane (providing SiH) in the presence of platinum-containing catalysts under heating is also known in the art. The resulting fully cross-linked materials display high rigidity and brittleness, a relatively high Tg, very high temperature resistance, water insensitivity and oxidation resistance. Examples of these types of polymers are found in U.S. Pat. Nos. 4,902,731 and 4,877,820.
Attempts have been made to reduce the brittleness and rigidity and increase the toughness of such polymers. U.S. Pat. No. 5,171,817 discloses such an organosilicone polymer in which reactive siloxane elastomers having carbon-carbon double bonds are added to the composition to form discontinuous phases in the rigid continuous polymer matrix after curing. For example, unsaturated diphenyl dimethyl siloxane elastomers are utilized to increase toughness and adhesion without reducing any of the other properties of the polymer. U.S. Pat. No. 5,196,498 discloses the use of a second silicone as a modifier to reduce the viscosity and brittleness of the crosslinked polymers. The second silicone compound, which has reactive hydrocarbyl group, is a cyclic siloxane and most preferably tetravinyltetramethyl cyclotetrasiloxane or pentavinylpentamethyl cyclopentasiloxane.
Accordingly, it would be advantageous for a polymer to have certain properties of known polymers without the brittleness and high rigidity that are usually associated therewith. It would be further advantageous for the polymer to have high acid and fuel resistance such that it would be suitable for use in fields which require polymers having more flexibility.
One potential use of such polymers is in the automotive industry as an encapsulant for items such as sensors, and especially as an encapsulant for pressure sensors. Such encapsulants must have an extremely high resistance to acids and fuels. For example, two fuels commonly utilized for material evaluation by the automobile industry, reference Fuel C and reference Fuel CM85, both cause polymer degradation to many known polymers. Fuel C is a hydrocarbon fuel which is approximately 50% by volume isooctane and 50% by volume toluene. Fuel CM85 comprises Fuel C containing 85% by volume methanol. In addition many materials are tested by the automotive industry for their resistance to used synthetic oil, which also causes polymer degradation. For the purpose of this present application, this is defined as MOBIL 1 OIL, which has been used to lubricate an automotive engine for a minimum of 3,000 miles.